Stereoscopic 3D projection displays, also called 3D projection displays in this specification, need to display 2D images that represent both the left- and right-eye perspective views for viewers. This can be achieved, for example, by displaying a left-eye 2D image in one polarized light and a corresponding right-eye 2D image in orthogonally polarized light so that a viewer wearing polarizing glasses receives the correct 2D images in each eye and thus perceives a 3D image. This type of 3D display is known as a passive 3D display using polarization because the viewer can use simple polarizing glasses as opposed to active shutter glasses that open and close in synchronization with the appearance of the left and right eye images. One way to achieve passive 3D viewing by polarization is to use the so-called Z-screen™ with a single projector that uses un-polarized light, such as the projectors based on Texas Instruments DLP™ panels. In this case the left and right eye images are displayed sequentially and the Z-screen encodes them in orthogonally polarized light. In a 3D projection display with Z-screen, the light efficiency for 3D images is only about 12% compared to 100% for 2D images if no Z-screen is used in the same projector. In addition, 3D projectors with Z-screen can only be based on expensive 3-chip DLP projectors. Another approach to projecting 3D images is to use two projectors that simultaneously display the left- and right-eye images in orthogonally polarized light through two external polarizers. In dual projector stereoscopic 3D displays the light efficiency for 3D images is also low, about 19% compared to 100% for 2D images if both projectors were used for projecting 2D images without the external polarizers. Dual 3D projectors are very difficult to align and to operate. They can not be aligned well enough to show identical 2D images simultaneously, thus they are dedicated to display 3D images only. Another approach to projecting 3D images is to use two projectors that simultaneously display the left- and right-eye images in different color sets known as the Infitec™ approach and as disclosed in U.S. Pat. No. 6,867,775B2. The viewer wears passive color filter glasses that transmit a left-eye image in a first color set and a right-eye image in a second color set. Each color set consists of red, green and blue light and the two color sets do not overlap spectrally. The dual projector Infitec™ approach also has low light efficiency for 3D images, about 14% compared to 100% for 2D images if both projectors were used without the required left- and right-eye filter sets. Spectral misalignment between the left-eye and right-images reduces the color gamut for this approach, although a polarization preserving screen is not required.
In another type of stereoscopic 3D projection display, known as active 3D, a single projector can alternatively display right- and left-eye 2D images time sequentially. A viewer, wearing active liquid crystal (LC) shutter glasses that are synchronized with the projector, will be able to see 3D images because the left- and right-eye LC shutters open only when the correct eye image is displayed. The LC shutters typically transmit only about 35% of the incident light. In addition, because the left- and right-eye images are each displayed at most half of the time, this arrangement is very light inefficient for displaying 3D images, with a light efficiency of 16% compared to 100% for displaying 2D images without using the LC shutter glasses. The 3D images appear very dim in comparison to the same projector displaying a single 2D image. Furthermore, LC shutter glasses need electrical power to operate, making them bulky and inconvenient to wear and are more expensive than simple polarizing glasses. But an advantage of active 3D is that ordinary screens or curved screens can be used which are desirable in some applications.
Stereoscopic 3D projection systems using two reflective liquid crystal micro-display panels (also called liquid crystal on silicon LCOS) with high performance polarizing beam-splitters were disclosed in U.S. Pat. No. 5,982,541. In the disclosure, passive 3D mode with polarizing 3D glasses is realized by projecting left- and right-eye images having orthogonal polarizations from the two panels respectively. Full color displays are achieved by using color sequential schemes: one color at a time. At any given time, the two panels share the same colors, thus the stereoscopic projectors are not as efficient as more expensive six panels stereoscopic displays, also disclosed in the same patent.
Projection displays for displaying 2D images with two panel micro-display panels were disclosed in several prior art U.S. Pat. Nos. 6,995,738 B2, 6,650,377 B2, and 5,921,650, to enhance the brightness of 2D projectors. In some approaches in the prior art, one panel is used for one dedicated color in one polarization, such as red, the other panel is used for two other colors time sequentially in the orthogonal polarization, such as blue and green. This approach is to compensate for the red color deficiency in most of arc lamps used in projection displays. In those approaches, each panel alone is not capable to display full color images, but two panels in combination can form full color images. Furthermore, additional polarization rotation filters and clean-up polarizers are used in the output image beam. Thus, these approaches are not capable of displaying full color left- or right-eye images with orthogonal polarizations that are essentially required in stereoscopic 3D images by polarization.
In another approach disclosed in U.S. Pat. No. 6,995,738 B2 as shown in FIG. 1 (also disclosed in U.S. Pat. No. 6,650,377 B2), a projector with two LCOS panels for 2D displays and two color switches as the ones available from the company ColorLink, and an analyzer were disclosed in which at any given time two colors with different polarizations are incident onto the two panels respectively. Thus, it is in principal twice as efficient as a single panel display. However, in practice, the gain in efficiency is diminished because of several factors: the required use of polarized light, un-polarized light can not be converted to polarized light with 100% efficiency, the conversion efficiency is usually less than 80%; the absorption and spectral band reduction occurring in color switches which may consist of multiple birefringent layers or filters with active control through multiple transparent electrodes, a single color switch usually has a maximum transmittance of about 70%, two color switches in series have a transmittance about 49%. In addition, because these approaches must use a second color switch and an analyzer in the output image beam to absorb light in one undesirable color, they are not capable of displaying right-eye and left-eye images with orthogonal polarizations because the image light from both panels are in the same polarization.